The field of the invention relates to air/fuel control systems for vehicles equipped to run on a blend of alcohol and gasoline fuels.
Air/fuel control systems are known which trim fuel delivered to an engine in response to a comparison of an exhaust gas oxygen sensor (EGO sensor) to a reference value associated with stoichiometric combustion. As shown in FIGS. 1A and 1B, when the EGO sensor output (12) is rich of the reference value (14), a lean correction is generated (high value for signal EGOS in FIG. 1B). When the EGO sensor output (12) is lean of the reference (14), a rich correction is generated (low value of signal EGOS in FIG. 1B).
However, when the fuel includes alcohol (ethanol or methanol), the EGO sensor output is shifted in a direction lean of stoichiometry (as indicated by dashed line 16 in FIG. 1A). Accordingly, the feedback correction signal (EGOS in FIG. 1B) will switch at a value other than stoichiometry resulting in air/fuel operation lean of stoichiometry.
U.S. Pat. No. 4,945,882 attempts to compensate for this lean shift by shifting the reference value (for example, by shifting the reference voltage from line 14 to dash line 18 in FIG. 1A). The inventor herein has recognized a problem with such shifting of the reference voltage. More specifically (as shown at the intersection of dashed lines 16 and 18 in FIG. 1A), the reference comparison would occur during a relatively flat portion of the EGO signal where relatively small changes in output voltage correspond to relatively large changes in air/fuel ratio. A resulting problem is that such a comparison would be more highly susceptible to signal noise adversely affecting air/fuel control.